This invention relates generally to one piece plastic beverage bottles and more particularly to an improved bottle of this type which has flat horizontal surfaces of increased area that firmly support the bottle in an upright position when filled.
A major difficulty with the use of plastic bottles for carbonated beverages is the strength of the bottle base. Due to internal carbonation pressures which can exceed 75 psi, plastic bottles have a tendency to bulge outward at the base, resulting in what is referred to as a "rocker" which will rock back and forth when standing on a flat surface or possibly tip over. In addition, as the base bulges outward, the volume of the bottle increases, thereby lowering the fill line such that consumers may believe the bottle was not properly filled or sealed.
One solution to the problem of bulging is to provide a bottle having a hemispherical base portion and attach thereto a base cup having a flat lower surface for supporting the bottle in an upright position. This type of bottle is commonly referred to as a composite bottle. Composite bottles are widely used for carbonated beverage bottles of sixteen ounces or more. Increasing material cost for base cups has, however, encouraged the development of one piece bottles having a self-supporting base which is reinforced to prevent bulging due to carbonation pressures.
Several factors must be taken into consideration in the evaluation of a bottle base. Stability is one of the most important factors. A bottle must be stable when empty as well as full. An empty bottle must be stable enough to stand upright on bottle filling machinery. If bottles fall over while being conveyed, the efficiency of the filling operation will be adversely affected. To provide a stable bottle, the diameter of the bottle engaging area which contacts a supporting horizontal surface must be maximized. In addition, the area of the base that is in surface-to-surface contact with the supporting surface should be maximized.
Another evaluation factor is the strength of the base in resisting bursting upon impact when the bottle is filled. Stress cracking in the base will reduce the strength, resulting in a base which bursts easily. The amount of stress cracking is related to the base geometry. Relatively large radius curves in the base will reduce the stress cracking compared to a base with small radius curves.
Yet another evaluation factor to consider is the ability to properly vent the mold cavity when blow molding the bottle. Sufficient venting must be provided to ensure that the plastic material will be blown completely into each leg of the base to form feet at the lower ends of the legs which define the support surface engaging areas of the bottle.
Examples of one-piece containers are illustrated in the following patents: U.S. Pat. Nos. 3,598,270 entitled "Bottom End Structure for Plastic Containers"; 4,294,366 entitled "Free-Standing Plastic Bottle"; 4,318,489 entitled "Plastic Bottle"; 3,871,541 entitled "Bottom Structure for Plastic Containers"; 4,249,667 entitled "Plastic Container With a Generally Hemispherical Bottom Wall Having Hollow Legs Projecting Therefrom"; 4,261,948 entitled "Method of Increasing the Wall Thickness of a Bottom Structure of a Blown Plastic Material Container"; 4,254,882 entitled "Plastic Pressure Bottle; 4,301,933 entitled "Synthetic Resin Thin Walled Bottle"; 3,403,804 entitled "Blown Bottle of Flexible Plastics"; 3,727,783 entitled "Non-Everting Bottom for Thermoplastic Bottles"; 3,759,410 entitled "Pressure Resistant Plastic Container"; 3,811,588 entitled Bottle; 3,881,621 entitled "Plastic Container with Non-Everting Bottom"; 3,935,955 entitled "Container Bottom Structure"; 4,355,821 entitled "Blow Molded Plastic Material Bottle Bottom"; and 4,368,825 entitled "Self Standing Bottle Structure".
Each of these prior art bottles, however, has at least one disadvantage associated with its base structure. The base structure of a plastic bottle will deflect downwardly when the bottle is filled with a carbonated liquid. When this occurs in several of the bottles disclosed in the above patents, the diameters and sizes of the support surface engaging areas are decreased resulting in bottles of reduced stability when full.
Several of the bottles disclosed in the above patents also have base geometries with small feet and relatively small radius curves. This results in support instability and stress cracks which reduce the strength of the base causing the base to burst upon impact.
Most blow molded bottles are molded in mold cavities which are vented using pin hole vents at the lower end of the mold cavity. It has been found that pin hole vents are not large enough to provide sufficient venting to ensure that the plastic is moved completely into each of the bottle feet to consistently form the support surface engaging areas of the bottle.
With these short comings in the prior art in mind, it is an object of this invention to provide a bottle with flat support surface engaging areas of increased size and increased diameter relative to the bottle diameter to thereby improve stability of the filled bottle.
It is a further object of this invention to provide a bottle wherein deflection of the base due to filling does not decrease the support surface engaging area.
Accordingly it is a further object if the invention to provide a container with larger radius bends and curves to reduce the possibilities for stress cracking.
It is a further object of the invention to provide a method for making the bottle which enables the venting capacity of the mold to be effectively increased to enable the bottles to be consistently blow molded with the plastic drawn completely into the foot cavities at the lower end of the base.